A method of classifying crystalline substances on the basis of their unit cell. There are seven crystal systems. If the cell is a parallelopiped with sides a, b, and c and if α‎ is the angle between b and c, β‎ the angle between a and c, and γ‎ the angle between a and b, the systems are:

1. (1) cubic a=b=c and α=β=γ‎=90°

2. (2) tetragonal a=b≠c and α=β=γ=90°‎

3. (3) rhombic (or orthorhombic) a≠b≠c and α=β=γ=90°‎

4. (4) hexagonal a=b≠‎c and α=β=γ=90°‎

5. (5) trigonal a=b≠‎c and α=β=γ≠90°‎

6. (6) monoclinic a≠b≠c and α=γ=90°≠β‎

7. (7) triclinic a=b=c and α≠β≠γ‎